Charmonium spectroscopy has seen a revival over the past year or so, with various groups reporting heavy charmonium states (see CERN Courier January 2004 p9). The BaBar collaboration has now added to the list of new states, after a recent study of the initial state radiation process e+e− → γ π+ π− J/Ψ across the charmonium mass range.
The data were collected with the BaBar detector at the SLAC PEP-II asymmetric-energy e+e− storage ring, representing a luminosity of 233 fb−1 at a centre-of-mass energy slightly above 10 GeV. Candidate
J/Ψ mesons were reconstructed via their decays to e+e− and μ+μ−. BaBar observed an excess of 125 ± 23 events centred at a mass of about 4.26 GeV/c2, signifying the presence of one or more previously unobserved JPC = 1− states containing hidden charm.
At the current level of statistics the number of new states cannot be distinguished and the signal is compatible with a single resonance about 90 MeV/c2 wide, although the single-resonance fit probability is low. For the moment the particle has been named Y(4260).
The BaBar Collaboration 2005 http://arxiv.org/abs/hep-ex/0506081.